Environ Earth Sci (2012) 66:1385–1392
DOI 10.1007/s12665-011-1348-6
ORIGINAL ARTICLE
Tracing groundwater discharge in a High Arctic lake
using radon-222
Hilary A. Dugan • Tom Gleeson • Scott F. Lamoureux
Kent Novakowski
•
Received: 27 October 2010 / Accepted: 3 September 2011 / Published online: 16 September 2011
Ó Springer-Verlag 2011
Abstract In the High Arctic, groundwater fluxes are
limited by the presence of continuous permafrost, although
it has been hypothesized that there may be localized
groundwater flow and hydraulic connectivity beneath large
lakes, due to the presence of taliks, or large regions of
unfrozen ground. However, due to the logistical difficulty of
employing seepage meters and piezometers in deep, icecovered lakes, relatively little is known about groundwater
discharge to polar lakes. One method of assessing groundwater discharge is through the use of geochemical tracers.
We conducted a pilot study to quantify groundwater discharge into a High Arctic lake using dissolved radon gas as
a geochemical tracer. Lake water was collected in 15 L
polyvinyl chloride (PVC) bags with minimal atmospheric
interaction from a 25-m deep lake near Shellabear Point,
Melville Island, Northwest Territories, Canada. Sample
bags were aerated through a closed water loop for 60 min to
allow sufficient radon to equilibrate in a coupled air circuit.
Radon in air concentrations were measured on a Durridge
RAD7 portable alpha spectrometer. The field trial in a
H. A. Dugan (&)
Department of Earth and Environmental Sciences,
University of Illinois at Chicago, Chicago, IL 60607, USA
e-mail: [email protected]
T. Gleeson
Department of Civil Engineering, McGill University,
Montreal, QC H3A 2K6, Canada
S. F. Lamoureux
Department of Geography, Queen’s University,
Kingston, ON K7L 3N6, Canada
K. Novakowski
Department of Civil Engineering, Queen’s University,
Kingston, ON K7L 3N6, Canada
remote setting and separate tests with groundwater samples
collected from a temperate site demonstrate the utility of the
methodology. The limited results suggest that radon levels
in the lower water column are elevated above background
levels following nival melt in the surrounding watershed.
Although these results are insufficient to quantify groundwater discharge, the results suggest subsurface flow may
exist, and further study is warranted.
Keywords Groundwater Arctic lake Hypersaline Radon-222 Permafrost
Introduction
In the Arctic, relatively little is known about the hydrogeological processes of groundwater flow and routing (Woo
et al. 2008). Yet, groundwater is an important resource that
can influence geotechnical infrastructure such as pipeline
and roadway stability, determine the chemical signature of
surface water bodies, and impact a range of biological
processes including nutrient transport and fishery dynamics
(van Everdingen 1990). In most Arctic hydrological budgets, groundwater is assumed negligible due to the presence
of continuous permafrost, which restricts flow due to the
extremely low hydraulic conductivity of frozen ground (van
Everdingen 1990). However, in the High Arctic, lakes and
large ponds generate a thermal unconformity at the ground
surface due to the high heat capacity of water (Burn 2005).
Water above 0°C at the bottom of a lake will prevent lake
bottom sediments from freezing, initiating a sub-lacustrine
talik that may extend to the base of the permafrost (Burn
2002). Because of this unusual thermal regime, large lakes
may receive groundwater discharge, similar to lakes in
temperate regions (Winter 1999). If correct, groundwater
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may moderate water-level fluctuations, and supply nutrients
and inorganic ions to these lake ecosystems (Hayashi and
Rosenberry 2002).
Despite the potential importance of groundwater for
Arctic lake systems and water budgets, it is usually not
considered due to the inherent difficulty in measuring discharge (Woo 1990). In temperate systems, groundwater flow
into lakes is typically measured with seepage meters
installed in the bottom sediments (Lee 1977). In Arctic lakes,
where ice cover complicates sub-surface moorings, and
logistics often prohibit underwater diving, the installation of
seepage meters becomes prohibitive. Furthermore, the
localized nature of seepage measurements requires a spatial
sampling pattern to obtain a representative flux (Burnett
et al. 2003). By contrast, the use of geochemical tracers, such
as radon, naturally integrates conditions in the lake and
effectively eliminates the need for spatial measurements.
Radon (Rn-222) is an inert gas, with a half-life of
3.82 days that is produced during the radioactive decay of
radium-226, which has a half-life of 1,622 years. Since
radon is inert, and the first gaseous element in the uranium238 decay series, it will readily diffuse into air instead of
remaining bonded in the chemical compound that contained the parent element (Drever 2002). As radium is
present in all rock material, it will enter aqueous solution
during water–rock interactions. The processes of rock
weathering, radioactive decay of radium to radon, and
restricted atmospheric interaction, leads to a large concentration of unsupported radon in groundwater (Kraemer
and Genereux 1998). Conversely, surface waters typically
have low radon concentrations due to the negligible radon
content of precipitation. If any radon is present in surface
water, it will rapidly diffuse into the atmosphere. This
difference allows radon to function as an indirect tracer of
groundwater discharge into water bodies and any subsequent physical mixing processes.
With recent advances in detection methods, sampling
for radon has become more readily employed in hydrogeological studies (Lee and Kim 2006; Burnett et al. 2001;
Gleeson et al. 2009; Schmidt et al. 2009; Burnett and
Dulaiova 2003). In an effort to detect the presence of
groundwater discharge in a region of continuous permafrost, we conducted a pilot study to test if radon activity
could be used as evidence for groundwater discharge into a
coastal hypersaline basin in the Canadian High Arctic.
While the origin of hypersaline lakes in the Arctic has been
debated (Dugan and Lamoureux 2011), one explanation for
the accumulation of salt in the lake is the discharge and
accumulation of groundwater brines (Ouellet et al. 1989).
Although recent work has suggested that the period of tidal
influence during isolation of the lake from the ocean is also
a viable mechanism for brine accumulation (Dugan and
Lamoureux 2011), groundwater discharge was assessed
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Environ Earth Sci (2012) 66:1385–1392
using radon as a tracer to test alternative models. We
present a viable field method to conduct radon measurements in remote field settings, and generate an initial
dataset to evaluate discharge in this Arctic setting. To our
knowledge, this study was the first of its kind in the High
Arctic and adds to the sparse limnological literature using
radon as a tracer for groundwater discharge (Schmidt and
Schubert 2007; Kluge et al. 2007; Schmidt et al. 2009).
Study site
Shellabear Lake (unofficial name, 74°500 N, 113°300 W) is a
seasonally isolated marine basin located near Shellabear
Point on Melville Island in the Canadian High Arctic
(Figs. 1, 2). The lake, which is situated at sea level with a
1 m deep outlet, has an area of 0.59 km2 and a volume
8.49 9 106 m3. From November to June, the outlet is
obstructed by snow and ice, and the lake is isolated from
the ocean. However, open water conditions from June to
October permit marine inflow into the lake during the
summer. The lake is hypersaline, with a water column
salinity of 56 g kg-1.
The lake basin is relatively flat, with a maximum depth
of 26 m and a mean depth of 14 m. From approximately
June to October, the lake receives water input from the
ocean, and from one major river that drains a 17 km2
watershed. During the winter and until June, the lake is
hydrologically isolated from the watershed and atmosphere
by a 2 m thick ice cover. In 2009, fluvial inflow began on
16 June, and marine connection did not occur before 1 July
(Dugan and Lamoureux 2011).
Geological mapping around Shellabear Lake is limited
to regional studies and reconnaissance surficial mapping.
The area is underlain by the Devonian Griper Bay formation which is composed of subhorizontal units of sandstone, siltstone and shale. Surficial sediment cover is
composed of Quaternary glacial and marine sediments, but
the thickness and internal structure at Shellabear Point is
unknown (Hodgson et al. 1984).
Methods
In a simple lake system, there are three sources and two
sinks of radon. Radon is introduced into the system through
groundwater advection, diffusion from the sediments, and
in situ production, and is removed by atmospheric loss and
in situ decay (Schmidt and Schubert 2007). The objective
when utilizing radon as a tracer of groundwater flow is to
discern the advection term in the system.
At Shellabear Lake, a field camp was established from
1st to 26th June 2009. To measure radon concentrations,
Environ Earth Sci (2012) 66:1385–1392
Fig. 1 a Location of Shellabear Lake on Melville Island, in the
Canadian High Arctic. b Shellabear Lake watershed (from NTS
map 88 E/15, 1:50,000). Areas of polygons are noted (PG).
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c Bathymetry of Shellabear Lake and location of middle station
where sampling took place. Bathymetric isolines and contour lines are
at 6 and 10 m intervals, respectively
Fig. 2 A photograph of Shellabear Lake looking north toward Liddon Gulf, Arctic Ocean, taken 13 June 2009
a Durridge RAD7 alpha spectrometer was employed in
conjunction with a Durridge RAD AQUA. The intrinsic
background of the RAD7 is 0.2 Bq m-3, and the calibration accuracy is 5% (Durridge 2000). The high
resolution capability of this detector makes it well suited
for a system where low radon concentrations are
expected. In addition, the RAD7 was chosen because it
allows sample analysis directly in the field. This is a key
advantage in a remote environment where samples cannot be returned to the laboratory in less than 1–4 weeks
and liquid scintillation analysis is not possible (Zouridakis
et al. 2002).
Due to time constraints in the field, the water column
was only sampled for radon at a station located at the
central, deepest site (Middle Station, Figs. 1, 2). Sampling
took place from the ice cover on 4th, 5th, 11th, 18th, 19th,
24th, and 25th June at water column depths of 2, 6, 12, 20,
and 26 m. In addition, samples from the river gauging
station, approximately 100 m upstream from the lake and
the river/lake confluence, were obtained on 24th, 25th and
26th June. Finally, background atmospheric levels were
measured on 11th, 23rd and 26th June.
Samples were collected from the lake at depth with a
2-L Kemmerer water sampler. The spigot on the outflow of
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Environ Earth Sci (2012) 66:1385–1392
the Kemmerer was attached to a short section of PVC
tubing connected directly to a 15-L PVC bag to avoid
atmospheric contamination. The Kemmerer was repeatedly
lowered to the required depth, and each sample collected
was withdrawn directly to the 15-L bag until the bag was
full. The bag was then returned to a nearby shore station
which housed the RAD7. The PVC bags were chosen to
take advantage of a larger sample size, which improves the
minimum detectable radon activity (Stringer and Burnett
2004). To analyse samples, the bag was positioned at
height and allowed to drain through the RAD AQUA,
which aerated the water. The bottom of the RAD AQUA
was equipped with a Micra Plus submersible pump, which
returned water back to the 15-L bag, creating a closed loop
water circuit. The air intake was positioned at the side of
the RAD AQUA, and ran through a desiccant before
reaching the RAD 7. The air loop was not purged with
radon-free air prior to sampling, as it was decided that
background atmospheric levels were sufficiently low that
atmospheric radon would not contaminate results. An
Onset HOBO external temperature probe was attached to
the RAD AQUA to record the temperature of the aerated
water. Due to the limited number of available PVC bags,
samples were discarded following analysis. Because secondary radon analysis was not preformed, the concentration of radon in the water column supported solely from
background radium-226 cannot be distinguished from
groundwater supplied radon, or the ‘excess radon’.
Near the end of June, when the river inflow was near peak
discharge, samples were analyzed from the lake inlet, and
from the river. At this time, no 15 L bags remained intact, so
samples were collected in 40 mL vials which allow direct
use with the RAD H2O apparatus. This sample size is
intended for higher concentration samples (Durridge 2000).
Each sample was analyzed for at least 1 h, with measurements taken every 10 min to permit adequate time for
the sample to equilibrate in the air. Samples were always
analyzed within 1.5 h of collection. For each sample, the
results from the first 30 min were discarded, and the final
readings and uncertainties were averaged for final concentrations in air. A partition coefficient ‘k’ for each
sample was calculated from:
k ¼ 0:105 þ 0:405eð0:0502TÞ
ð2Þ
where Vair represents the volume of air in the closed air
loop between the RAD H2O and the sampling bag, and Vw
is the volume of water in the sampling bag.
123
To validate the methodology, groundwater samples were
collected near Kingston, Ontario. Three, 15 L bags were
filled with groundwater from the same well on 28 July
2009. One bag was analysed on each of 28 July, 30 July,
and 1 Aug, using identical methods to those performed in
the field. The decay of any radioisotope can be modeled
with a standard decay equation:
C ¼ Co ekt
ð3Þ
where C is the new concentration of the isotope, Co is the
concentration at the time of sampling, k is the decay constant, and t is time in days. The decay constant is calculated
based on the species’ half-life. For radon, k = ln (2)/
3.8235 = 0.181286. Results show that radon in the bags
decayed as predicted by the radon decay curve (Fig. 3),
which indicates a lack of atmospheric interaction in the test
system. This result confirms that the sampling methodology was sound, and results can be considered accurate.
Without some knowledge of local radon levels in
groundwater, it remains impossible to calculate groundwater fluxes. Unlike temperate systems where samples are
drawn from nearby wells, continuous permafrost restricts
below-ground drilling. This is the foremost challenge in
quantifying groundwater flow in Arctic systems. Therefore,
the goal of this paper was not to quantify groundwater
discharge, but to establish if probable evidence could be
demonstrated for its existence.
ð1Þ
where T is the temperature recorded from the external
temperature probe. The concentration of radon in water
(Cw) was then calculated from the concentration in air
(Cair) following (Lee and Kim 2006):
Cw ¼ ðCair Vair þ k Cair Vw Þ=Vw
Fig. 3 Radon concentration in groundwater samples allowed to
decay over 4 days. The decay curve matches the theoretical decay
calculated from a radon-222 half-life of 3.8235 days
Results
The concentration of radon in the atmosphere at Shellabear
Lake was tested on three separate occasions, and all results
reveal atmospheric levels at 0 Bq m-3. Atmospheric background radon levels are likely negligible due to sedimentary
bedrock and the lack of local human atmospheric influence.
These results permitted analysis of water samples without
purging the air lines with radon-free gas beforehand.
Environ Earth Sci (2012) 66:1385–1392
1389
Fig. 4 Radon concentration of
water samples from Shellabear
Lake. Error bars represent
analytical uncertainties
associated with the RAD7. The
grey box represents the
hypothesized maximum
concentration of background
radium in the water column (see
text)
Table 1 Radon concentrations of river and lake inlet samples
Date
Location
Concentration
(Bq m-3)
Uncertainty
(Bq m-3)
24 June 2009
Lake inlet
65.2
4.7
25 June 2009
Lake inlet
38.0
3.8
25 June 2009
River station
44.4
7.7
26 June 2009
Lake inlet
38.4
5.7
26 June 2009
River station
34.2
4.3
Radon concentrations in the lake were measureable, and
higher than background air levels. Samples ranged from
5 to 30 Bq m-3, with the majority of samples falling
between 5 and 20 Bq m-3. While a 1–2 m surface ice
cover was present for the entirety of sampling, which
prevented atmospheric degassing, elevated radon concentrations were not discernable in the epilimnion. Prior to
sampling on 24th June, the only discernable depth or
temporal pattern was a approximately linear increase in
radon concentrations at 25–27 m (Fig. 4). On 24th and
25th June, the mean radon concentration in the water column was elevated as compared to the previous sample
dates, largely due to a single high radon concentration at
12 m depth. The radon levels measured in the lake inlet
and at the river station were higher than those recorded
in the lake. The highest recorded concentration was
65.2 Bq m-3 and was obtained at the river outlet on 24th
June (Table 1).
Discussion
To the best of our knowledge, using radon as a tracer in the
High Arctic has not been attempted prior to this study. This
limits any hypotheses that can be derived based on similar
environments. However, limnological radon studies have
been conducted in temperate locations using similar
methodologies (Table 2). From uncorrected data, it appears
that the concentration of radon in Shellabear Lake is
slightly lower, but near the levels documented in other
studies. Although results were not corrected for background radium, the levels are expected to be similar to
other lake studies or lower, due to the clastic sedimentary
geology of the region (Hodgson et al. 1984). Therefore, we
do not expect background radium to be [* 20 Bq m-3.
Diffusion of radon from the bottom sediments is not considered significant as samples from 25 to 27 m depth were
not consistently higher than the remainder of the water
column.
The water column in Shellabear Lake is hypersaline
below a chemocline at 5 m depth. Due to the high stability
of the bottom water, advective mixing by wind is negligible, especially when a surface ice cover is present
(9 months of the year). Similarly, mixing from river inflow
is minimized due to the strong density contrast between the
epilimnion and the hypersaline hypolimnion. Therefore, if
a lake-bottom groundwater seep is present, a relatively
shallow diffusion profile would be expected. Prior to 24th
June, the relatively constant radon concentrations in the
water column may signal that radon production is largely
supported by background radium. On June 24th, the mean
radon concentration in the water column increases. If
background radium is assumed to be B20 Bq m-3, the
16–20 m sample on 25th June is substantially higher than
background levels. We consider this rise in radon concentration on 24 and 25th June to be excess radon, which
may indicate the presence of groundwater discharge in
Shellabear Lake. The excess radon in the upper water
column may be associated with the linear increase in radon
concentration in the bottom water samples throughout the
study period.
123
123
Schmidt et al. (2010)
Study site
0.11
Area (km2)
Below detection
0.94
Ra226 (Bq m-3)
Atmospheric radon (Bq m-3)
9.3
N/A
E
11.2–36.6
July
31
0.89
2.5
1,694,600
1.91
6.6
3.8
6.9E
14.8H
29.3H
42.8H
19.7E
17.7H
19.3E
Dec
666
37.7E
Oct
777
4.5
7,200
0.0034
Moraine of Tertiary
lignite, clay, silt and
sand layers
Schmidt and
Schubert (2007)
Lake Waldsee:
Germany
and H, respectively
Shale and silty shale
bedrock overlain
by glacial till
Lake B: Alberta,
Canada
Samples taken from the epilimnion and hypolimnion are denoted by
July
Max. specific conductivity
(lS cm-1)
0.5–1.1
21
Zmean (m)
Radon (Bq m-3)
1
Zmax (m)
Sampling month
112,300
2
Vol (m )
3
Fine grained sandstone,
overlain by glacial drift
and fluvial sediments
Surficial geology
Lake A: Alberta,
Canada
Temperate
Climate
5.7
18.6
57.3
Apr
780
10
24
560,000
0.53
2.8
11.1
57.8
May
700
Unconsolidated glacial
till. Mainly silt, sand
and gravel layers
Schmidt et al. (2009)
Lake Ammelshainer:
Germany
Table 2 Comparison between the physical characteristics and radon concentrations of four temperate lakes and Shellabear Lake
7.4
B4
2.6–52.4
July
900
8
20
1,300,000
0.145
Aquifer consisting
of Quaternary sand
and gravel
Lake Willersinnweiher: Germany
Kluge et al. (2007)
0.0
N/A
5.4–30.0
June
80,000
14.4
25
8,490,000
0.59
Quaternary glacial and
marine sediments underlain
by sedimentary bedrock
Shellabear Lake:
NWT, Canada
This study
Polar
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Environ Earth Sci (2012) 66:1385–1392
While the highest radon levels were concurrent with the
onset of melt and river formation on 16th June, we propose
that the river is not the source of excess radon for two
reasons. First, the inflowing river water is freshwater, and
is restricted to a thin layer between the ice cover and the
underlying hypersaline water (Dugan and Lamoureux
2011). Therefore, if the river is supplying excess radon,
only the surface water should be enriched. This is contrary
to what was observed in Shellabear Lake, as the 2 m
sample on 24th June was lower in concentration than the 12
and 27 m samples. Secondly, the overall river discharge
was low, reaching a maximum of 0.76 m3 s-1 on 19th
June. The quantity of river inflow in proportion to lake
water is insufficient to raise radon concentrations of the
larger lake volume. However, the difference in sampling
methods between lake and river samples must also be taken
into account, which limits comparisons, especially at such
low overall concentrations.
The increase in radon levels at 25–27 m depth may
provide evidence for groundwater input at depth. If there is
sub-lacustrine discharge to Shellabear Lake, it is expected
to occur following melt, when liquid water is prevalent in
the system. Under these conditions, surface water that
infiltrates a local groundwater supply could increase the
hydraulic gradient and may initiate groundwater discharge.
If a talik exists below the lake, water will be directed into
the bottom of the lake (Fig. 5). A heuristic rule is that a
1391
lake will be underlain by an open talik if the minimum
horizontal width of the lake is twice the permafrost thickness in nearby terrain (McGinnis and Jensen 1971). At
Winter Harbour, Melville Island (80 km east of Shellabear
Lake), temperature measurements obtained from a 3750-m
oil exploration well situated 1.2 km inland from the coast
indicate a permafrost depth of 557 m (Brown 1972). The
width of Shellabear Lake ranges from 800 to 1,000 m and
therefore there is a high probability of an open sublacustrine talik. Furthermore, permafrost measurements are
based on a 0°C boundary. In an area of saline sediments,
freezing point depression due to high salinities can contribute to groundwater flow below 0°C (Pollard et al.
1999).
At the lake bottom, discharge is often restricted due to
the accumulation of thick sediments (Fig. 5). If sediments
are predominately silts and clays, the low hydraulic conductivity will restrict inflow (Schmidt et al. 2010). Instead,
discharge may enter the lake from the sides, a common
feature in temperate lakes (Kluge et al. 2007; Winter 1999).
There are two explanations that may account for the
limited range of radon concentrations observed in June
2009. First, sampling was only conducted in the centre of
the lake. It may be that discharge enters the lake on the
sides, as observed by Kluge et al. (2007), where the radon
concentration in the small, shallow basins was relatively
high versus sampling undertaken at the deepest location. By
the time radon diffuses to the middle of the lake, most
excess radon has decayed. Secondly, melt did not begin
until mid-June, and the landscape was still partially snowcovered and cold at the end of June. Therefore, it is likely
that sampling was conducted too early to discern sustained
radon inflow into the lake, and later in the season, when the
active layer is thicker and liquid surface water may penetrate into aquifers, more groundwater flow may be expected.
However, these explanations are not mutually exclusive and
it is clear that the 2009 sampling regime was limited.
Expansion of sampling throughout the lake, as well as
throughout the melt season will supply more robust results.
Conclusions
Fig. 5 A hypothesized cross-section of the hydrological characteristics of Shellabear Lake (not to scale). Note the river inflows, the outlet
to the ocean, and an open sublacustrine talik. Within the section of
unfrozen ground, groundwater discharge may enter the water column
at the edges of the basin where low permeability lake sediments are
thin or non-existent
The goal of this study was to identify the presence of
groundwater flow into a coastal lake in the High Arctic.
Shellabear Lake was chosen due to the high brine content,
but this methodology can be applied to any lake system.
Based on limited data, radon concentrations increased
above stable background levels at select depths in the water
column after the onset of snowmelt and river runoff. This
may denote the presence of sub-lacustrine discharge into
the lake. Due to limited sampling, the relative contributions
of groundwater discharge, background decay of radium,
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and radon diffusion from lake sediments cannot be
elucidated.
These results raise numerous questions surrounding the
presence of groundwater flow in Arctic systems and a more
thorough evaluation of radon concentrations in Shellabear
Lake and lakes throughout the Arctic is required. Additional sampling near shore and through the melt-season is
warranted and would improve the understanding of the
origin and flow characteristics of groundwater through
continuous permafrost.
Acknowledgments This research would not have been possible
without funding from the National Science and Engineering Research
Council (NSERC), excellent logistical support from the Polar Continental Shelf Program (PCSP), and field assistance from Kasey Kathan. Thank you to T. Kluge and A. Schmidt for providing
descriptions of surficial geology for their study sites. This is PCSP
contribution # 01116.
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